When the king of the mermaids is poisoned, Lily is called to help with a problem she's never faced before—one beneath the surface of the water. As she sets about creating a potion to save his life, however, she realizes this is only one part of a much much larger problem. One morning, Lily receives a letter from an anonymous source stating that The Faction is planning to launch an attack against The Isle. Can Lily stop her father and The Faction before it's too late?